1. Field of the Invention
The present invention relates to an apparatus and method of manufacturing an optical fiber preform and, more particularly, to a means for detecting the weight of a rotatably supported preform during rotation of the preform.
2. Related Background Art
As a conventional method of manufacturing an optical fiber preform, a VAD (Vapor Axial Deposition) method or an OVD (Outside Vapor Deposition) method is generally known. In the VAD or OVD method, fuel gases such as H.sub.2 and O.sub.2 and material gases such as SiCl.sub.4 and GeCl.sub.4 are supplied to a burner in a reaction vessel, and these gases are burned to generate fine particles of SiO.sub.2 and GeO.sub.2. Then, the fine glass particles are deposited on a rotating target.
A target used in the VAD method is a preform itself grown from the distal end of a starting rod. In the OVD method, a preform grown on a core rod and in the periphery thereof is used as a target. In the VAD method, the preform is gradually fed in the axial direction to grow the preform in the axial direction. The OVD method is different from the VAD method in that the preform and the burner are relatively moved to grow the preform in the radial direction. In the VAD method, normally, the preform is vertically arranged, and the starting rod is vertically pulled upward, thereby growing the preform. Also in the OVD method, the preform is sometimes grown in a vertically arranged state.
In the VAD or OVD method, normally, the growth state of the preform is continuously monitored and fed back to parameters for changing the growth rate, e.g., the flow rates of the fuel and source gases supplied to the burner, the pull-up speed of the preform, and the exhaust rate of the reaction vessel.
As a method of monitoring the growth rate of a preform, a preform growth point monitor by a laser is normally used. However, a method of monitoring the weight of a preform is disclosed in Japanese Patent Laid Open No. 59-45936.
FIG. 1 is a view showing a conventional apparatus for manufacturing an optical fiber preform as disclosed in Japanese Patent Laid Open No. 59-45936. The apparatus shown in FIG. 1 uses the VAD method to manufacture a preform and can monitor the weight of the preform. In this apparatus, a seed rod 1 is supported by a chuck 3 through a load cell 2. The lower portion of the seed rod 1 extends into a reaction vessel 4. In the reaction vessel 4, the upper end of a starting rod 5 is connected to the lower end of the seed rod 1. A preform 6 is grown from the lower end of the starting rod 5 in the vertical direction, i.e., in the axial direction of the starting rod 5. The load cell 2 is used to measure the weight of the preform 6.
The chuck 3 is attached to an elevating unit (not shown) and can be vertically moved. The chuck 3 is rotatably supported and connected to a motor (not shown). Therefore, the seed rod 1, the load cell 2, the starting rod 5, and the preform 6, all of which are suspended from the chuck 3, can be rotated and vertically moved. The load cell 2 outputs a signal corresponding to the weight of the growing preform 6 to a controller 7. The controller 7 sends the signal representing the weight of the preform to mass flow controllers 8 to 11, thereby adjusting the flow rates of H.sub.2 and O.sub.2 to a burner 12, and at the same time adjusting the flow rate of Ar as a carrier gas for bubbling SiCl.sub.4 and GeCl.sub.4 gases, i.e., adjusting the flow rates of the SiCl.sub.4 and GeCl.sub.4 gases to the burner 12. The controller 7 can also adjust the degree of opening of an exhaust valve 14 provided in an exhaust pipe 13 of the reaction vessel 4, thereby adjusting the exhaust amount.
In the above-described conventional preform manufacturing apparatus, the growth state of the preform 6 can be continuously monitored by detecting the weight of the preform by the load cell 2. However, the flexural rigidity of the load cell 2, i.e., the rigidity against a force in the radial direction, is relatively low. For this reason, in the above arrangement in which the load cell 2 is rotated together with the seed rod 1 and the preform 6, the distal end of the preform 6 tends to offset from the original rotation axis of the seed rod 1 during rotation.
Since the normal growth rate of the preform 6 is several grams per minute, sensitivity of the load cell adapted for this order is required. To realize this sensitivity, the sensor portion of the load cell 2 must have a low flexural rigidity to detect a very small amount. Additionally, to prevent a measurement error due to friction, the seed rod 1 cannot be restrained in the radial direction. Therefore, the above-described conventional apparatus has a structure easily affected by offset of the preform 6. Offset of the preform 6 causes an eccentric growth. As a result, when an optical fiber is finally manufactured from such a preform, the optical fiber core may undesirably become eccentric, or non-circularity of the fiber is generated.
In addition, in the above-described apparatus, a signal from the load cell 2 must be extracted using a slip ring (not shown) serving as a movable contact. This tends to cause noise inclusion in the signal.